The term ‘dimming’ may be generally understood as driving a load at a power that is lower than its rated power. In the case of a lamp, this reduction in power is immediately noticeable as a reduction in light output, hence the origin of the term ‘dimming’. Other appliances can also be ‘dimmed’, for example an electric motor can be driven at a lower power by turning a ‘dimmer’ knob or switch. Such a knob or switch may be incorporated into the appliance or lamp, for example a desk-top lamp may have its own dimmer knob. A ceiling light fixture may be dimmed by a rotatable knob realized as part of the wall switch for that lighting fixture. The power of a hand-held appliance can be regulated by a knob incorporated in a housing of the appliance. Known dimmers that are used in appliances that run off a mains supply usually comprise an AC switch component such a phase-controlled triac, and are generally referred to as ‘leading edge dimmers’ or ‘forward phase’ dimmers. Such a dimmer effectively suppresses the early part of each sinusoidal half-wave of AC voltage to the load, by triggering the triac at a certain phase angle. The dimming level is generally controlled by a user input such as a rotatable knob whose action is converted into the trigger signal for the triac.
Instead of a triac, a component such as an alternistor, or silicon-controlled rectifier (SCR) can be used as the AC switch, with an appropriate control signal. A triac or alternistor is a semiconductor component that can conduct electric current in both directions when triggered. A silicon-controlled rectifier SCR is a similar device that conducts current in only one direction when triggered. For AC operation, two SCRs are arranged in an anti-parallel configuration. Triac/alternistor and anti-parallel connected SCRs are functionally identical and are referred to collectively in the following to as an ‘AC switch’. For an AC switch to start conducting, the load current must exceed a ‘latching current’ before the end of the trigger pulse. Once latched, the AC switch will remain in the conductive state until the load current once again drops below a ‘holding current’ level. Latching current and holding current values are specific for each AC switch and are typically in the order of few tens of milliamperes.
Leading edge dimmers are traditionally used for dimming of incandescent lamps, which present an inherently resistive load to the dimmer. This type of load has unity power factor and generally draws enough current to ensure stable AC switch operation over the full dimming range. A resistive or linear load is associated with nearly perfect AC switch commutation, i.e. the transition between non-conductive and conductive states, which in turn ensures smooth operation over the whole dimming range. Even low-power incandescent lamps draw enough current when dimmed so that even if the load current should drop below the holding current level, any distortion is imperceptible. Similar is true in the case of an inductive load such as a magnetic transformer used to supply a low-voltage halogen lamp. In this case, the load current may drop to zero after the mains voltage zero-crossing point.
However, in the case of a light capacitive or non-linear load such as an electronic LED (light-emitting diode) driver or converter, the situation is more complex. Various undesirable issues may arise, for example missing cycles, a sudden change in output voltage or current, irregular AC switch turn-off at the end of a mains cycle, etc., and lead to unacceptable system performance. The type of problem will depend to a large extent on the nature of the load, the source impedance, the AC switch triggering method and various other circuit parameters.
A simple workaround for such an unacceptable performance is to add an extra load, commonly referred to as a ‘bleeder’, ‘minimum load’ or ‘dummy load’, to ensure correct AC switch commutation. A bleeder circuit assists dimmer operation by making up any difference between the load current and latching/holding currents, and by compensating the reactive nature of the load. Bleeders can be either linear or non-linear, and can comprise electronic circuitry with various levels of complexity. An advantage of linear bleeders is in their simplicity and low cost, but they produce a relatively significant amount of heat, typically 25-60 W, which can be a problem in some applications. Maintenance costs may also be significant, for example when an incandescent lamp with limited lifetime is used as the extra load. The prior art bleeders use the voltage or current at the dimmer output, so that the bleeder is realized on the load side of an arrangement comprising dimmer, load and bleeder. Usually, the prior art bleeder circuits are physically incorporated in the load, so that the dimmer itself is an entity that is electrically connected to but physically separate from the combined load and bleeder. This adds to complexity and cost in the design and manufacture of dimmable electrical appliances. Such dimmer circuit is for instance described in US2011/0291583. This dimmer circuit comprises a dimmer between AC mains and a rectifier. A DC signal is provided to a load. Between the rectifier and the load a bleeder and a phase detect circuit is provided. The phase detect circuit detects a phase angle of the rectified output signal generated by the dimmer. A programmable micro controller is coupled to the phase detect circuit and also receives the output signal generated by the dimmer. The micro controller generates a control signal to turn on/off the bleeder.
Design of a non-linear bleeder, also known as an ‘active load’, is based on the premise that extra load is needed only when the AC switch current drops below a holding current level. Compared to a linear bleeder, a functionally equivalent active load can reduce power dissipation by an order of magnitude. Active loads are generally also maintenance-free. However, active loads are significantly more complex and therefore more expensive than their linear counterparts. Another type of a non-linear bleeder is the ‘switched resistor bleeder’, which performs better than a linear bleeder and is less complex than an active load. A switched resistor bleeder effectively switches a power resistor across the dimmer output when the mains voltage drops below a certain threshold level. The extra load is added to the output of the dimmer only when it is needed, so that the overall power dissipation is less when compared to a linear bleeder. Even though a switched resistor is relatively simple in construction, it typically dissipates more power than a functionally equivalent active load.
Light-emitting diodes (LEDs) are becoming more widespread, and are being used more often to replace incandescent and halogen light sources. Many of the LED lamps available on the market today are retrofit for existing incandescent lamps. Because of its higher luminous efficiency, the power of an LED light source is only about one fifth of that of an equivalent incandescent lamp, so that an LED lamp can be regarded as a ‘light load’, whereby the term ‘light’ refers in this context to its low power consumption (in contrast to an incandescent lamp such as a 70 W light bulb which may be regarded as a ‘heavy’ load). However, when only a few such low-power LED lamps are controlled by a dimmer, the low power consumption typically results in latching issues at both ends of the mains half cycle. For example, an AC switch of the dimmer may ‘overshoot’, i.e. it may continue to conduct beyond the voltage zero-crossing point. Such an overshoot is undesirable since it may be perceived as annoying lamp flicker and can generate an undesirable DC current in the load and consequently also in the mains. Furthermore, an LED lamp generally always requires some kind of electronic driver. A part of every LED driver is a power converter, usually a switched-mode power converter, so that an LED lamp is effectively a non-linear load. Therefore, from the point of view of a dimmer, an LED lamp effectively presents a ‘light non-linear load’.
Therefore, it is an object of the invention to provide an improved way of dimming a light non-linear load, avoiding the problems mentioned above.